(1) Field of the Invention
The present invention relates to a process for producing a polycarbodiimide resin. More particularly, the present invention relates to a process for producing a polycarbodiimide resin having good storage stability and excellent miscibility with organic solvents.
(2) Description of the Prior Art
Polycarbodiimide resins having a structure of --N.dbd.C.dbd.N-- in the molecule are known to have high heat resistance and other excellent properties and are in use in various fields as a thermosetting resin.
Polycarbodiimide resins have been produced in a reaction system using a solvent, except for few exceptional cases, because since the reactivity of carbodiimide group is high, carbodiimidization in high degree invites gelling due to the occurrence of side reactions. Consequently, the resulting polycarbodiimide resin has had limited applications owing to the solvent used.
For example, a polycarbodiimide resin produced using a high-boiling solvent finds no applications where a low-boiling solvent must be used.
The above polycarbodiimide resin may be used in said applications by conducting concentration to remove part or all of the high-boiling solvent and replace it with a low-boiling solvent. However, since the reactivity of carbodiimide group is so high as to invite gelling even in a solution, it is virtually very difficult to remove part or all of the high-boiling solvent.
Even if the above problem caused by the use of a solvent in polycarbodiimide production could be solved, problems still exist when the produced polycarbodiimide resin has low storage stability or poor miscibility with organic solvents.